In tape drive systems where tape is driven by a belt engaging the periphery of the tape supply and take-up rolls, the velocities of the belt in the take-up and supply regions are unequal and hence unbalanced in such a manner that tension is developed in the tape span between the belt/tape-roll engagement regions.
Various methods of driving tape with peripheral belt drive transports give rise to various velocity differentials in the drive belts. Some of these methods are exemplified in U.S. Pat. Nos. 3,305,186 to Burdorf; 3,620,473 to Stone; 3,692,255 to VonBehren and 4,172,569 to Newell. Each of these patents teaches a means of developing a velocity differential at the belt radial centerlines; however, the resulting velocity at the tape radial centerlines in the corresponding tape engagement regions is reduced by the radius ratios of belt centerlines to tape centerlines. The velocity differentials of belt and tape radial centerlines will be equal only when the supply and take-up roll radii are equal, or at the midpoint of tape passage.
The problem of net belt tension variation in stiff-belted drives was first defined mathematically by W. E. Seaman and later described in a paper entitled "An Improved ANSI-Compatible Magnetic Tape Cartridge" by C. W. Newell, published in the IEEE Transactions on Magnetics, Vol. Mag.-14 No. 4, July 1978. For practical tape hub and roll radii, tape thicknesses and tape drive geometry, using belts of conventional design as to the thickness and elasticity, it was shown that the tape tension variation which must be tolerated is typically on the order of 25% of nominal tape tension, or higher.
An object of the invention is to devise a method for forming seamless metal rings to make very thin, yet stiff belts for minimizing the net tension variation in the drive belt, i.e. the so-called Seaman effect.
In stiff-belted drives of the type described in the previously mentioned U.S. Pat. No. 4,172,569 a unique problem has arisen, first identified by R. T. Steinbrenner of Bell Telephone Labs, and for purposes of identification known as the "Steinbrenner effect." As the tape is drawn onto the tape take-up roll periphery, it is initially wrapped in loose contact with the roll.
As the belt wraps upon the take-up roll periphery, the driving inner surface of the belt will lose length with respect to the belt centerline, if the belt and tape webs are allowed to slip on the periphery while the belt is being bent around the take-up roll. This loss in length or "shrinking" at the belt surface will be accompanied by an incremental increase in the surface velocity of the belt where it first engages the tape. The resultant tape velocity in the slipping interface between roll and belt will also incrementally increase during the slip, since the first tape layer is loosely wrapped on the roll. The larger the slip angle, the greater the amount of extra tape that will be drawn in at the nip. The slip angle can be reduced by: (a) reducing the amount of air entrapped between the first and second tape layers; (b) increasing the coefficient of friction between the belt surface and the tape; the inner surface velocity will thus very quickly match the tape roll surface, and all belt stretch will be positive and outward from the inner belt surface rather than positive and negative around the belt centerline.
It should be noted that the incremental increase in tape velocity at the take-up nip is not complemented by an incremental decrease of tape velocity at the supply nip, since the tape is already "wrung" to the surface of the supply roll periphery and thus is metered solely by the tape-roll periphery itself and not by the belt, per se. The above phenomena result in an "inch-worming" effect, whereby upon reversal of tape, a small but significant increment of tape is drawn in, and an incremental tape tension increase will result. Upon repeated reversals over the same tape length, the tension will climb until the forces build to equilibrium, the tape in effect pulling itself out of the nip at the same rate it is being pulled in.
A second object of the invention therefore is to devise a method for forming stiff metal rings for belts which will allow entrapped air to readily escape, and further to provide such belts with a high-friction surface, so that the slip angle will be reduced essentially to zero, minimizing the Steinbrenner effect.
In stiff-belted drives of the type where the belt wraps in contact with the tape recording surface, as previously described, yet another problem arises. That problem is physical damage to the tape surface from wear induced by the drive belt. It is important to avoid scuffing of the oxide surface, to minimize the generation of loss oxide particles.
It is also important to avoid embossing of any such particles into the surface of the tape, to minimize the likelihood of their being impact-welded to the tape surface before they can drop harmlessly away or can be actively removed by cleaners, whichever the case may be. Of particular concern are particles which are loosened in the scuffing action at the take-up nip between belt and tape roll, since these will be embossed on the tape surface by the belt before they can either drop away or be removed.
Therefore, the drive belt should make soft contact with the tape in order to substantially eliminate embossing of loose oxide particles into the recording surface, thereby substantially increasing the life of the tape as to digital data dropout rates or analog signal-to-noise ratio.